An important function of lubricant compositions, and in particular gear and axle lubricant fluids, is to provide a high degree of reliability and durability in the service life of equipment in which it is installed. Lubricating oils in general, and gear and axle lubricants in particular, frequently must satisfy a relatively large number of performance criteria to be commercially successful. For example, a commercially successful axle lubricant will frequently be required to possess a high degree of oxidative stability, compatibility, shear stability, corrosion avoidance or resistance, wear protection, shiftability, and extended drain. Beyond this, it is desirable for a high viscosity lubricant composition to meet low temperature performance requirements. These properties represent a difficult to achieve set of performance criteria.
Gear lubricant compositions are classified by the American Petroleum Institute (“API”) using “GL” ratings. These classifications are subdivided into six classes. The lowest rating, API GL-1, classifies oils used for light conditions, which consist of base oils without additives. The highest rating, API GL-6, classifies oils for very heavy conditions, such as high speeds of sliding and significant shock loading, and which contain up to 10% high performance antiscuffing additives. However, class API GL-6 is not applied any more as it is considered that class API GL-5 will meet most severe requirements. Lubricant compositions classified meeting API GL-5 performance requirements are generally applied, for example, in hypoid gears having significant displacement of axles.
The viscosity-temperature relationship of a lubricating composition is another of the critical criteria to be considered when selecting a lubricant for a particular application. Mineral oils commonly used as a base for single and multigraded lubricants exhibit a relatively large change in viscosity with a change in temperature. Fluids exhibiting such a relatively large change in viscosity with temperature have a low viscosity index. The Society of Automotive Engineers (“SAE”) publication SAE J306 describes viscometric qualifications for axle and gear lubricant compositions. This classification is based on the lubricant viscosity measured at both high and low temperatures. The high-temperature kinematic viscosity values are determined according to ASTM D 445, with the results reported in centistokes (cSt). The low-temperature viscosity values are determined according to ASTM D 2983 and the results are reported in centipoise (cP). These two viscosity units are related as follows in Equation 1:(cP/(Density,g/cm3))=cSt  (Eq. 1)
The following Table 1 summarizes high and low temperature requirements for qualifications of axle and gear lubricant compositions.
TABLE 1Maximum TemeratureSAE Viscosityfor Viscosity ofViscosity at 100° C., cStGrade150,000 cP, ° C.MinimumMaximum7OW −554.1—75W−404.1—80W−267.0—85W−1211.0— 80—7.0<11.0 85—11.0<13.5 90—13.5<18.5110—18.5<24.0140—24.0<32.5190—32.5<41.0250—41.0—
These SAE standards are intended for use by equipment manufacturers in defining and recommending automotive gear, axle, and manual transmission lubricants, for oil marketers in labeling such lubricants with respect to their viscosity, and for users in following their owner's manual recommendations.
A lubricant composition's viscosity may be defined in two ways: (1) based on its kinematic viscosity; or (2) based on its apparent (Brookfield) viscosity. Kinematic viscosity is defined as a lubricant composition's resistance to flow and shear due to gravity. Apparent viscosity relates a lubricant composition's resistance to flow and shear due to internal friction.
High temperature viscosity is related to the hydrodynamic lubrication characteristics of the fluid. Some lubricant compositions may contain high molecular weight polymers, known as viscosity modifiers or viscosity index improvers, which function to increase the viscosity of the fluids. During use, however, these polymers may shear to a lower molecular weight, thereby resulting in a fluid with a lower viscosity than that of the new fluid. Low temperature viscosity requirements are related to the ability of the fluid to flow and provide adequate lubrication to critical parts under low ambient temperature conditions.
Lubricating compositions meeting SAE viscosity grade 75W-110 are known. For example, EP 1191090 discloses lubricating compositions comprising (a) from about 40-70% by weight of mineral oil, (b) from about 2-20% by weight of vinyl aromatic-diene copolymers, olefin copolymers, and mixtures thereof, (c) from about 2-20% by weight of a high viscosity polyalphaolefin, and (d) from about 3-20% by weight of a gear additive package.
Applicants have come to recognize, however, that although a substantial number of lubricant compositions have been produced having various needed properties where such lubricant compositions are used, there exists a need for lubricant compositions comprising API Group III mineral base oils that provide improved high viscosity lubricant compositions meeting low temperature performance requirements. While acceptable performance of the gear oil is a requirement, it is also highly desirable that the lubricant compositions be low in cost and easily produced. Accordingly, there is a need in the art for a lubricant composition that meets these industry standards and further provides cost-effective alternatives that may be easily produced, and in particular lubricant compositions classified as SAE 75W-110 meeting GL-5 performance requirements and having an apparent viscosity of less than about 150,000 cP at −40° C.